Interaction effect between electromigration and microstructure evolution in Cu/Sn-58Bi/Cu solder interconnect

The interaction effect between electromigration (EM) and microstructure evolution in Cu/Sn-58Bi/Cu solder interconnect with asymmetric configuration along current direction was studied by cellular automaton (CA) modeling embedded with finite element (FE) simulation, with a comparison to in-situ scanning electron microscope (SEM) observation and focus ion beam (FIB) microanalysis. The CA method was proposed to simulate the eutectic structure of Sn-58Bi solder alloy. Results show that the accumulation of Bi-rich phase tends to occur near the bottom corner position of the interconnect owing to the smallest local electrical resistance and severest current crowding effect. At the bottom corner of the interconnect, the current density in Sn-rich phase is much higher than that in Bi-rich phase. Bi atoms in Sn-rich phase are more prone to migrate to the anode first, rather than migrating directly along the interface between Bi-rich phase and Sn-rich phase. By employing the criterion of EM induced atomic flux of Bi in FE analysis under CA rules, simulation results of Bi-rich phase segregation are consistent with the experimental observation under current stressing.